Influence of the Structure on the Electrochemical Performance of

Tuning Li-Ion Diffusion in α-LiMn1–xFexPO4 Nanocrystals by Antisite Defects and Embedded β-Phase for Advanced .... Chemistry of Materials 0 (proof...
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Chem. Mater. 2001, 13, 1570-1576

Influence of the Structure on the Electrochemical Performance of Lithium Transition Metal Phosphates as Cathodic Materials in Rechargeable Lithium Batteries: A New High-Pressure Form of LiMPO4 (M ) Fe and Ni) O. Garcı´a-Moreno,† M. Alvarez-Vega,† F. Garcı´a-Alvarado,† J. Garcı´a-Jaca,‡ J. M. Gallardo-Amores,‡ M. L. Sanjua´n,§ and U. Amador*,† Departamento de Quı´mica Inorga´ nica y Materiales, Facultad de Ciencias Experimentales y Te´ cnicas, Universidad San Pablo-CEU, Urbanizacio´ n Monteprı´ncipe, 28668 Boadilla del Monte, Madrid, Spain, Laboratorio de Altas Presiones, Facultad de Ciencias Quı´micas, Universidad Complutense, 28040 Madrid, Spain, and Instituto de Ciencia de Materiales de Arago´ n, Universidad de Zaragoza-CSIC, Facultad de Ciencias, 50009 Zaragoza, Spain Received July 24, 2000. Revised Manuscript Received February 6, 2001

Materials built from MO6 octahedra linked to XO4 tetrahedra are good candidates for studying the different factors that determine the electrode potential. Among them, olivinelike LiMPO4 (M ) transition metal) phosphates are especially interesting. When pressure is applied to LiMPO4 (M ) Ni and Fe), a phase transition is induced. However, instead of the well-known olivine S spinel transformation, a transition to a new phase is observed (β′). The arrangements of the metal ions (including phosphorus) in the two structures are very similar; thus, the main difference between them is due to the oxygen arrangement in a similar matrix. Raman spectroscopy has confirmed the structural model proposed for the high-pressure phase, in particular the modification in the lithium coordination from 6- to 4-fold upon synthesis under pressure. Among the olivines LiMPO4 (M ) Mn, Ni, and Fe), the iron-containing one is only active up to 5.1 V. On the other hand, none of the highpressure materials is electrochemically active; this can be explained by the change in the electrostatic field at the transition metal position.

Introduction During the past few years the search for new cathode materials for rechargeable lithium batteries has been mainly focused on lithium transition metal oxides. Among them, the most useful or promising are those with a layered rock-salt structure or spinel-like structure.1-6 The latter compounds show the highest voltage of a working redox couple found so far;5,6 high oxidation voltage is necessary to obtain high specific energy in a battery constructed with a given anode. Thus, in the design of new cathodic materials, an understanding of the chemical and the structural features that control the energy of the active redox couple, and as a consequence the voltage, is of crucial importance. The energy of a given redox couple varies from one material to another depending on two main factors: (1) the elec* Corresponding author. Telephone: 34 91 372 47 37. Fax: 34 91 351 04 75. E-mail: [email protected]. † Universidad San Pablo-CEU. ‡ Universidad Complutense. § Universidad de Zaragoza-CSIC. (1) Broussely, M. J. Power Sources 1999, 81-82, 137. (2) Broussely, M. J. Power Sources 1999, 81-82, 140. (3) Kodama, T.; Sakaebe, H. J. Power Sources 1999, 81-82, 144. (4) Owens, B. B.; Smyrl, W. H.; Xu, J. J. J. Power Sources 1999, 81-82, 150. (5) Kawai, H.; Nagata, M.; Tsukamoto, H.; West, A. R. Electrochem. Solid-State Lett. 1998, 1, 212. (6) Fey, G.; Li, W.; Dand, J. R. J. Electrochem. Soc. 1994, 141, 2279.

trostatic field at the cation position and (2) the covalent contribution to the cation-anion bonding. The study of both factors can be properly accomplished on oxides formed by MO6 octahedra (M ) transition metal cation) linked by tetrahedral polyanions (XO4n- ) PO43-, AsO43-, SO42-, MoO42-, etc.). The electrostatic field changes when the structure does, whereas the covalence of the M-O bond differs in isostructural compounds with different tetrahedral polyanions by inductive effect due to the different degrees of covalence in the X-O bond.7 Among the above compounds, olivine-like phosphates are extremely interesting, not only from a basic point of view, but also for their possible use as cathodic materials. In this connection, LiCoPO4 olivine has very recently been reported to be a high-voltage/high-energy electrode.8 In addition to the preparation of a new form of LiMPO4 (M ) Fe and Ni) materials obtained under high pressure from the corresponding olivine-like compounds and the characterization of the two structures by Raman spectroscopy, in this paper, the electrochemical behavior of both series of compounds is presented and discussed in connection with their structures. (7) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J. Electrochem. Soc. 1997, 144, 1188. (8) Amine, K.; Yasuda, K.; Yamachi, M. Electrochem. Solid-State Lett. 2000, 3 (4), 178.

10.1021/cm000596p CCC: $20.00 © 2001 American Chemical Society Published on Web 04/13/2001

New High-Pressure Form of LiMPO4 (M ) Fe and Ni)

Chem. Mater., Vol. 13, No. 5, 2001 1571

Table 1. Final Structural Parameters of LiMPO4 Olivine-like Compounds M ) Mna atom site Li M P O(1) O(2) O(3)

4a 4c 4c 4c 4c 8d

symmd

x/a

-1 .m. .m. .m. .m. 1

0 0.2826(5) 0.096(1) 0.099(2) 0.456(2) 0.157(1)

y/b

z/c

0

0 0.972(1) 1/ 0.413(1) 4 1/ 0.732(5) 4 1/ 0.216(3) 4 0.047(3) 0.274(2) 1/

4

M ) Feb Biso

(Å2)

1.7(5) 0.6(2) 0.5(2) 0.8(3) 0.8(3) 0.8(3)

x/a 0 0.2857(5) 0.0943(7) 0.094(3) 0.451(1) 0.167(1)

y/b

M ) Nic

z/c

0

0 0.9737(8) 1/ 0.4265(5) 4 1/ 0.746(1) 4 1/ 0.193(2) 4 0.050(3) 0.300(2) 1/

4

Biso

(Å2)

1.1(4) 0.8(2) 0.4(1) 0.9(3) 0.9(3) 0.9(3)

x/a 0 0.2760(3) 0.0945(6) 0.092(1) 0.450(1) 0.1624(9)

y/b

Biso (Å2)

z/c

0

0 0.9809(7) 1/ 0.420(1) 4 1/ 0.745(2) 4 1/ 0.219(2) 4 0.045(1) 0.276(2) 1/

4

0.69(2) 0.37(2) 0.37(2) 0.21(1) 0.21(1) 0.21(1)

a For M ) Mn, a ) 10.431(9) Å, b ) 6.0947(6) Å, c ) 4.7366(5) Å, V ) 301.12(3) Å3, F ) 3.46(1) g/cm3, R ) 0.079, R B exp ) 0.075, Rwp ) 0.21, χ2 ) 7.7. b For M ) Fe, a ) 10.227(1) Å, b ) 6.0048(7) Å, c ) 4.6918(7) Å, V ) 288.12(4) Å3, F ) 3.88(1) g/cm3, RB ) 0.057, Rexp ) 0.060, Rwp ) 0.22, χ2 ) 14. c For M ) Ni, a ) 10.0275(3) Å, b ) 5.8537(2) Å, c ) 4.6763(2) Å, V ) 274.49(1) Å3, F ) 3.88(1) g/cm3, RB ) 0.057, Rexp ) 0.087, Rwp ) 0.151, χ2 ) 2.95. d Symmetry group ) Pnma (62).

Experimental Section LiMPO4 (M ) Mn or Ni) compounds with olivine-like structures were prepared by the reaction of stoichiometric amounts of analytical-grade (Aldrich 99.99%) M(CO3), (NH4)2HPO4 and Li2CO3 under flowing argon. The samples enriched with 6Li used in the Raman experiments were prepared using 6LiOH‚H O (Aldrich) as the lithium source. To obtain single2 phase samples the thermal treatment was performed in three different steps: (1) 398 K for 24 h, (2) 498 K for 24 h, and (3) 1023 K for 48 h. After each treatment, the sample was ground and pelletized. To obtain a single-phase sample of the ironcontaining olivine, Fe3(PO4)2 must be used as the iron source, because, when iron carbonate or nitrate is used, some impurity phases appear. This Fe(II) phosphate was prepared by direct reaction of FePO4 and metallic iron in an evacuated quartz ampule at 1073 K for 24 h. Following the above procedure, pale orange, strong yellow, and gray samples were obtained for manganese, nickel, and iron, respectively. The high-pressure experiments were performed on a belttype press working at 65 kbar and 1173 K. In all cases, the high-pressure materials had a pale green color. The samples were determined to be single-phase by X-ray powder diffraction with a Siemens D501 apparatus using monochromatic Cu KR radiation. The X-ray diffraction data were analyzed using the program Fullprof.9 Thermal analyses were carried out to assess the chemical stability of the single-phase samples up to 872 °C on a Seiko 320U system. Experiments were run at heating rate of 10 °C min-1 (Pt/Pt-Rh thermocouple) in a dry air stream flowing at 100 mL min-1 (typical specimen weight of 10-12 mg). The Raman spectra were measured at room temperature in a Dilor XY spectrometer with a diode-array multichannel detector. Light from a Coherent Ar+ laser at 514.5 nm was focused onto the sample through a ×50 microscope objective lens. The light power at the sample was 10 mW, and the spot size was 4 µm2. The spectral resolution was typically 3 cm-1. The lithium ionic conductivity was measured on pellets of the samples by ac impedance spectroscopy using a Solartron 160 impedance analyzer over a frequency range from 20 Hz to 20 MHz and a temperature range from room temperature to 772 K. Both opposite faces of the pellets were pasted with platinum electrodes. The electrochemical experiments were performed in Swagelok cells with the following configuration: Li/LiPF6 (1 M) in EC (ethylene carbonate) + DMC (dimethyl carbonate) (1:1)/host material + carbon black + binder. Cells were cycled under potentiostatic conditions using (10 mV/0.5 h steps; the cell voltage was measured against the lithium metal electrode.

Results and Discussions Structural Study. From powder X-ray diffraction experiments, the samples LiMPO4 (M ) Mn, Fe and Ni) were found to be single-phase with an olivine-like structure. Figure 1 shows the result of fitting the (9) Rodrı´guez-Carvajal, J. FULLPROF, version 2.4.2; ILL (unpublished).

Figure 1. Results of fitting the X-ray powder diffraction data corresponding to olivine LiNiPO4: experimental (points), calculated (solid line), and difference (bottom). Table 2. Selected Interatomic Distances in Olivine-like LiMPO4 Compounds M ) Mn Mn-O(1) Mn-O(2) Mn-O(3) Mn-O(3) P-O(1) P-O(2) P-O(3) Li-O(1) Li-O(2) LI-O(3)

2.23(1) 2.14(2) 2.13(2) × 2 2.30(2) × 2 1.50(3) 1.59(3) 1.54(2) × 2 2.24(2) × 2 2.08(1) × 2 2.11(1) × 2

M ) Fe Fe-O(1) Fe-O(2) Fe-O(3) Fe-O(3) P-O(1) P-O(2) P-O(3) Li-O(1) Li-O(2) LI-O(3)

2.25(1) 2.00(1) 2.03(1) × 2 2.30(1) × 2 1.50(1) 1.58(1) 1.53(1) × 2 2.15(1) × 2 2.14(1) × 2 2.25(1) × 2

M ) Ni Ni-O(1) Ni-O(2) Ni-O(3) Ni-O(3) P-O(1) P-O(2) P-O(3) Li-O(1) Li-O(2) LI-O(3)

2.15(1) 2.07(1) 2.072(8) × 2 2.150(8) × 2 1.52(1) 1.59(2) 1.540(9) × 2 2.101(8) × 2 2.030(8) × 2 2.090(8) × 2

experimental X-ray diffraction pattern for LiNiPO4 using as starting model the structure proposed in ref 10; the difference between the observed and calculated data is also shown. The patterns corresponding to the other LiMPO4 olivine-like materials are similar. The final structural parameters for all compounds are collected in Table 1, and Table 2 shows some selected interatomic distances. In Figure 2a, a schematic representation of the olivine structure is depicted. This structure is usually described in terms of a hexagonal close-packing of oxygen with Li and M ions located in one-half of the octahedral sites and P in one-eighth of the tetrahedral positions. The MO6 octahedra share four corners in the cb plane being cross-linked along the a axis by the PO4 groups, whereas the Li ions are located in rows, running along a, of edge-shared LiO6 octahedra that appear between two consecutive [MO6]∞ layers described above.11 (10) Abrahams, I.; Durand, J. L. Acta Crystallogr. C 1993, 49, 925. (11) Megaw, H. D. Crystal Structures: A Working Approach; Saunders: London, 1973.

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Figure 2. Schematic representation of the structures of (a) olivine, (b) β′-LiNiPO4, and (c) CrVO4. Table 3. Final Structural Parameters of β′-LiMPO4 Compounds M ) Fea atom Li M P O(1) O(2)

site 4c 4a 4c 8f 8g

symmetryc m2m 2/m.. m2m m.. ..m

x/a 0 0 0 0 0.228(1)

M ) Nib

y/b 0.676(2) 0 0.3504(6) 0.247(1) 0.467(1)

z/c 1/

4

0 1/ 4 0.043(1) 1/ 4

Biso

(Å2)

1.8(6) 1.0(3) 1.2(4) 1.5(5) 1.5(5)

x/a 0 0 0 0 0.227(1)

z/c

Biso (Å2)

0 1/ 4 0.044(1) 1/ 4

1.5(6) 0.54(4) 0.62(4) 0.87(5) 0.87(5)

y/b 0.675(2) 0 0.3504(3) 0.246(3) 0.466(3)

1/

4

a For M ) Fe, a ) 5.5230(3) Å, b ) 8.2565(5) Å, c ) 6.1583(3) Å, V ) 280.82(5) Å3, F ) 3.73(2) g/cm3, R ) 0.055, R B exp ) 0.057, Rwp ) 0.25, χ2 ) 19. b For M ) Ni, a ) 5.3580(2) Å, b ) 8.1272(3) Å, c ) 6.1241(3) Å, V ) 266.7(1) Å3, F ) 4.00(2) g/cm3, RB ) 0.033, Rexp ) 0.028, Rwp ) 0.09, χ2 ) 5.7. c Symmetry group ) Cmcm (63).

Table 4. Selected Interatomic Distances in β′-LiMPO4 Compounds M ) Fe Fe-O(1) Fe-O(2) P-O(1) P-O(2) Li-O(1) Li-O(2)

Figure 3. Experimental (points), calculated (solid line), and difference (bottom) X-ray powder diffraction patterns for β′LiNiPO4.

When pressure is applied to the LiMPO4 (M ) Fe and Ni) olivine-like compounds, a phase transition is induced, whereas the manganese-containing olivine seems to be stable up to the limit of the experimental setup (65 kbar). However, instead of the well-known olivine S spinel transformation,12,13 a transition to a new phase is observed. The structure of this phase (called β′LiMPO4 hereafter), as determined by powder XRD is shown in Figure 2b. It is similar to that of CrVO4 (Figure 2c),14 with lithium ions located in empty tetrahedral positions. In Figure 3 the experimental X-ray diffraction data for β′-LiNiPO4, the calculated pattern using the structural model proposed in Table 3 and their difference are shown. The pattern corresponding to β′LiFePO4 is similar to that for β′-LiNiPO4. The final structural parameters for both compounds are also (12) Hornstra, J. J. Phys. Chem. Solids 1960, 15, 311. (13) Binns, R. A.; Davis, R. N.; Reed, S. B. J. Nature 1969, 221, 227. (14) Frazer, B. C.; Brown, P. J. Phys. Rev. 1967, 125, 1283.

2.06(1) × 2 2.17(1) × 4 1.53(1) × 2 1.59(1) × 2 1.91(1) × 2 2.14(1) × 2

M ) Ni Ni-O(1) Ni-O(2) P-O(1) P-O(2) Li-O(1) Li-O(2)

2.03(1) × 2 2.13(1) × 4 1.55(1) × 2 1.52(1) × 2 1.90(1) × 2 2.09(1) × 2

collected in Table 3, and Table 4 shows some selected interatomic distances. The structure of the β′-LiMPO4 (M ) Fe and Ni) compounds consists of rows of edge-sharing MO6 octahedra running along c that are cross-linked by LiO4 and PO4 tetrahedra along the a axis, giving rise to layers of composition [(MO6)(LiO4)(PO4)]∞ lying in the ac plane. These layers are stacked along the b axis with a displacement of a/2 (Figure 4). Alternatively, the structure can be described by the stacking, along the [100] direction, of mixed layers composed of MO6 octahedra and LiO4 and PO4 tetrahedra. In one layer, every MO6 octahedron shares two opposite edges with neighboring MO6 octahedra and two apical oxygen atoms with two LiO4 and two phosphate groups. A phosphate group shares two oxygen atoms with two different MO6 polyhedra and one edge with a LiO4 tetrahedron; in this way, the [MO6]∞ edge-sharing rows are linked along the [010] direction by two rows of composition [(LiO4)(PO4)]∞, condensed in such a way that every LiO4 tetrahedra in one row shares one edge with a phosphate in the second row (Figure 5). These layers are stacked along the a axis in such a way that the largest window in one layer is blocked on both sides by an octahedron of neighboring layers. This alternative description based on the polyhedra arrangement seems to be more suitable for determining structural relationships between this structure and the olivine one, as was previously done for the olivine S spinel transition.12,15

New High-Pressure Form of LiMPO4 (M ) Fe and Ni)

Chem. Mater., Vol. 13, No. 5, 2001 1573

Figure 4. Schematic representation of the stacking sequence of [(MO6)(LiO4)(PO4)]∞ layers along the b axis in β′-LiMPO4 (see text).

Figure 6. “Metallic” framework in (a) olivine- and (b) β′LiNiPO4. Figure 5. Depiction of the structure of β′-LiMPO4 by stacking of layers along the a axis (see text).

In this case both olivine and spinel are close-packed structures, with a hexagonal (ABAB) sequence in the former and a cubic (ABCABC) sequence in the latter, and relationships between the two structures can be established. In contrast, the oxygen ions are not closepacked in β′-LiMPO4, rendering this viewpoint less useful in describing the pressure-induced phase transition olivine S β′-LiMPO4. In this connection, it is worth noting that the arrangements of metal ions (including phosphorus) in the two structures are very similar. In olivine, Ni and P define distorted hexagonal sheets stacked along the b axis in a hexagonal sequence (ABAB), with lithium ions located in the center of distorted octahedra of phosphorus atoms (Figure 6a). The metal framework in the β′ phase is very similar, as can be seen in Figure 6b; in this case, the hexagonal sheets are formed by phosphorus and lithium, whereas the nickel ions occupy the octahedral sites defined by phosphorus. Although an important rearrangement is produced through the phase transition, the metallic frameworks in the two structures are almost the same, assuming that lithium and nickel can be interchanged. In this connection, the high-pressure phase is metastable at ambient pressure and transforms into olivine on heating (to allow for motion of the metallic ions) at about 697 K. As shown in Figure 7, when the metals (15) Choisnet, J.; Hervieu, M.; Raveau, B.; Tarte, P. J. Solid State Chem. 1982, 45, 280.

Figure 7. TGA/TDA data for β′-LiNiPO4.

are rearranged, the excess energy of the β′-phase is eliminated. It is well-known that several olivine-like compounds transform under high pressure into denser structures. Thus, the high-pressure form of the silicate Mg2SiO416 is about 6% denser than the olivine form; and some germanates, such as Ca2GeO4, evolve under high pressure to materials with a K2NiF4 structure that is about 25% denser, whereas others such as Mn2GeO4 transform to a Sr2PbO4-type structure (18% denser).17 It is worth pointing out that the volume reduction in the transition from the olivine to the β′ phase is only about 2.5%. Even more important, in general, phase transitions induced (16) Baur, W. H. Am. Mineral. 1972, 57, 709. (17) Reid, A. F.; Ringwood, A. E. J. Solid State Chem. 1970, 1, 557.

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Figure 8. Raman spectra of olivine LiNiPO4 with (a) natural lithium and (b) 100% 6Li.

Figure 9. Raman spectra of β′-LiNiPO4 with (a) natural lithium and (b) 100% 6Li.

by pressure yield to structures with higher coordination numbers (cn’s), or at least the cn’s of all atoms in the low- and high-pressure phases are the same (for instance in the olivine S spinel transition). To the best of our knowledge, no pressure-induced transition has been reported so far in which the coordination number of one type of cation is lowered. This is the case in the olivine S β′-LiMPO4 transition. In the olivine form of LiMPO4 compounds, the lithium ions are located in the middle of edge-sharing LiO6 octahedra. In the β′-phase, the lithium ions are located in unconnected LiO4 tetrahedra. This description of the lithium coordination in the two structures is obtained when only the nearest oxygen atoms are considered (Li-O distances up to 2.5 Å), as is the common practice in most cases. However, if the second coordination sphere is also taken into account, the coordination of lithium increases from that of the ambient to the high-pressure structure. In particular, for Li-O distances up to 3.2 Å, the coordination numbers are 6 in olivine and 4 + 4 in the β′-phase phase. Because this is a very important point and powder XRD is not the most suitable technique for studying structural effects related to light atoms, such as lithium, a spectroscopic study was performed to confirm the proposed structural model. Spectroscopic Study. We have measured the Raman spectrum of polycrystalline LiNiPO4 in both the olivine and the β′ phases. Compounds with 100% substitution of 6Li were also studied in order to verify the assignment of some modes to lithium vibrations. Figure 8a and b shows the Raman spectra of LiNiPO4 in the olivine phase (a) with the natural proportion of lithium isotopes and (b) with 100% 6Li. Figure 9a and b shows the corresponding spectra for the β′ phase. In Figures 8 and 9, all of the resolved peaks were above 100 cm-1; below this frequency, only a structureless background was found. These spectra can clearly be divided into two regions, above and below 400 cm-1, that we can approximately (except for the lithium modes) correlate with the domain of appearance of internal and external modes, respectively. Internal modes involve the displacement of oxygen atoms of the pseudo-tetrahedral (PO4)3- anions and present frequencies closely related to those of the free molecule (in solution, for instance) with perfect Td symmetry. For (PO4)3-, these are18 a singlet (A1) at a

Table 5. Factor Group Analysis for Olivine LiNiPO4

(18) Nakamoto, K. Infrared and Raman spectra of inorganic and coordination compounds; J. Wiley & Sons: New York, 1978.

Translations of Li, Ni, PO4: 36 modes atom

site

Ag

Au

B1g

B1u

B2g

B2u

B3g

B3u

Li Ni P

-1, 4a, Ci .m., 4c, Csxz .m., 4c, Csxz

2 2

3 1 1

1 1

3 2 2

2 2

3 1 1

1 1

3 2 2

Librations of PO4: 12 modes Td

site Csxz

correlation to D2h

Τ1

A′ + 2A"

Ag + B1u + B2g + B3u + 2Au + 2B1g + 2B2u + 2B3g Internal modes of PO4: 36 modes

Td

site Csxz

correlation to D2h

ν1 (A1) ν2 (Ε) ν3 (Τ2) ν4 (Τ2)

A′ A′ + A" 2A′ + A" 2A′ + A"

Ag + B1u + B2g + B3u Ag + B1u + B2g + B3u + Au + B1g + B2u + B3g 2Ag + 2B1u + 2B2g + 2B3u + Au + B1g + B2u + B3g 2Ag + 2B1u + 2B2g + 2B3u + Au + B1g + B2u + B3g

frequency ν1 ) 938 cm-1; a doublet (E) at ν2 ) 465 cm-1; and two triply degenerate (T2) modes, ν3 at 1027 cm-1 and ν4 at 567 cm-1. ν1 and ν3 involve the stretching of the P-O bonds, whereas ν2 and ν4 involve mostly O-P-O bending, with a small contribution of P vibration. In a solid, internal modes can split as a consequence of an electric crystal field of symmetry lower than tetrahedral acting on the molecule (site-symmetry effect) and also because of the presence of more than one molecular group in the crystal unit cell (correlation effect). External modes consist, on one hand, of Li, Ni, and PO4 translations and, on the other hand, of rotational oscillations (librations) of PO4 units. Librational modes can also show site-symmetry and correlation effects. According to this separation, factor group analysis gives the modes for LiNiPO4 with the olivine structure (space group Pnma) collected in Table 5. Among all of these modes, only the even ones (Ag, B1g, B2g, B3g) are Ramanactive. Thus, lithium atoms located in sites with inversion symmetry are not spectroscopically active, so that we do not expect to see modes involving lithium vibrations. As a whole, we expect 12, 6, and 18 Raman modes of translational, librational, and internal character, respectively. In the β′ phase (symmetry group Cmcm), the activity of each type of atom is shown in Table 6. Only the 18 even modes are Raman-active, including 6 translations, 3 librations, and 9 internal modes. In this phase, lithium atoms occupy a quasi-tetrahedral site and are Ramanactive. In contrast, Ni atoms, at a site of symmetry C2h, are inactive.

New High-Pressure Form of LiMPO4 (M ) Fe and Ni)

Chem. Mater., Vol. 13, No. 5, 2001 1575

Table 6. Factor Group Analysis for β′-LiNiPO4 Translations of Li, Ni, PO4: 24 modes atom

site

Ag

Li Ni P

m2m, 4c, C2vy 2/m.., 4a, C2hx m2m, 4c, C2vy

1

Au

B1g

B1u

B2g

B2u

B3u

1

1 2 1

1

1 2 1

1 1 1

1 1

1

1

Librations of PO4: 6 modes Td

site C2vy

correlation to D2h

Τ1

A2 + B1+ B2

Au + B1g + B1u + B2 g + B3g + B3u

Internal modes of PO4: 18 modes Td

site C2vy

correlation to D2h

ν1 (A1) ν2 (Ε) ν3 (Τ2) ν4 (Τ2)

A1 A 1 + A2 A1 + B1+ B2 A 1 + B1 + B2

Ag + B2u Ag + B2u + Au + B2g Ag + B2u + B1u + B3g + B1g + B3u Ag + B2u + B1u + B3g + B1g + B3u

Table 7. Assignment of the Experimental Bands of the Raman Spectra of Olivine and β′-LiNiPO4 Raman shifts Raman shifts (cm-1) 7LiNiPO4 (cm-1) 6LiNiPO4 mode attribution (olivine phase) 118.3 168.8 174.3 236.3 256.6 280.5 301.7 322.3 416.8 437.8 466 582.7 590.9 600 640 947.5 1010 1021 1071 1086

119.4 169.6 175.3 238.5 257.5 283 304 323.5 417 439 467.8 580.7 591 601 641.4 948.5 1011 1072.5 1088

Raman shifts Raman shifts (cm-1) 7LiNiPO4 (cm-1) 6LiNiPO4 185 251.1 258.5 342 455 497.5 565.4 622 648.7 912.8 940.5 1057.8 1149.7

189.4 250.5 256.5 364.8 453.5 522 565 625 654 913 939.4 1058 1148

phonon or PO4 libration phonon or PO4 libration phonon or PO4 libration phonon or PO4 libration phonon or PO4 libration phonon or PO4 libration phonon or PO4 libration phonon or PO4 libration ν2 ν2 ν2 ν4 ν4 ν4 ν4 ν1 ν3 double phonon or ν3 ν3 ν3 mode attribution (β′ phase) phonon or PO4 libration + lithium phonon or PO4 libration phonon or PO4 libration lithium ν2 lithium ν4 ν4 ν4 + lithium double phonon or ν3 ν1 ν3 ν3

With the help of Tables 5 and 6, the experimental bands can be assigned to specific vibrations. This is done in Table 7 for both phases of LiNiPO4. We shall first comment on the olivine phase data. In the region where internal modes of the phosphate anion appear, we identify the totally symmetric mode at ν1 ) 947.5 cm-1; the doublet ν2 between 416 cm-1 and 466 cm-1; and the triplets ν3 and ν4 in the regions 1010-1086 cm-1 and 580-640 cm-1, respectively. As expected, internal bands are split because of sitesymmetry and correlation effects. From these two factors, the former is expected to be dominant because the PO4 tetrahedron is considerably distorted. However, the appearance of more than two peaks in the ν2 region

and more than three in the ν4 region can only be due to correlation effects, as these frequencies are too high to be due to Ni or PO4 translations. The weak shoulder at 1020 cm-1 might be ascribed to a second-order peak. Below 400 cm-1, we expect the 12 translational modes of Ni and PO4, plus 6 librations of PO4 (only 3 if we neglect correlation effects). In any case, we expect more modes than are observed. There are some possible explanations for this discrepancy: some of the missing modes might be at frequencies below 100 cm-1 or unresolved in the experimental spectrum, or their Raman activity is just be too low to be detected. Modes of PO4 as whole, for instance, are likely to be unobserved. The mode attribution in this region might be done more accurately by measuring the temperature evolution of the spectrum or by partial substitution of Ni or PO4 by other transition metals or tetrahedral anions, respectively. Substitution of 7Li by 6Li in the olivine phase of LiNiPO4 does not result in any remarkable change in the Raman spectrum. The differences observed between the Raman shifts in 6LiNiPO4 and in 7LiNiPO4 are within the experimental error. This agrees with the fact that lithium is not Raman-active in this phase. In the β′ phase, we expect only 6 translational modes (3 from Li and 3 from PO4), 3 librational modes, and 9 internal modes. In this phase, there is no extra splitting of Raman bands by correlation effects, and in fact, the number of peaks appearing in the internal-mode region can be explained by the low-symmetry site of the PO4 anion. As in the olivine phase, we attribute the peak at 913 cm-1 in the β′ phase to second-order activity, because its frequency is quite low to be a component of ν3 and no correlation splitting of ν1 is expected. Moreover, this agrees with twice the ν2 value. If this attribution is correct, then the splitting of the degenerate internal modes is lower than predicted by symmetry: the triplet modes split into a singlet and a doublet, and the ν2 doublet does not split. This suggests that the actual symmetry of the PO4 group might be higher than C2v or close to a higher symmetry group. The assignment given in Table 7 is based on the same arguments as for the olivine phase. In the β′ phase, the availability of samples synthesized with the 6Li isotope has been used to ascribe the bands at 342 and 497 cm-1 to lithium-only vibrations, as the relation between these values and the corresponding ones in the 6Li compound (365 and 522 cm-1, respectively) is very close to the inverse-square-root-of-the-masses law. There are two other modes, at 185 and 648 cm-1, whose frequencies also increase with 6Li substitution (to 189 and 654 cm-1, respectively). Therefore, we attribute to them a mixed character, of translational plus librational type, for the 185 cm-1 mode, and of translational plus internal type, for the 648 cm-1 mode. As in the olivine phase, we detect fewer modes than expected in the low-frequency region. The detection of modes due to lithium translation confirms the structural hypothesis, in particular the modification in the lithium coordination from 6- to 4-fold upon synthesis under pressure. On the other hand, the detection of well-defined modes associated with lithium suggests that the lithium sublattice is static, at least at room temperature, which agrees with the conductivity results.

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Alvarez-Vega et al. Table 8. Electrostatic Field at the Atoms Positions in Olivine and β′-LiFePO4 LiFePO4 olivine

Figure 10. Electrochemical performance of olivine and β′LiFe4PO4.

Finally, we comment on the apparent lack of correlation between the experimental frequencies of the ν1 mode in the olivine and β′ phases (947 and 940 cm-1, respectively) and the length of the P-O bonds. Usually, the shorter bonds of the β′ phase should result in higher stretching frequencies. We might find the explanation, however, in the different coordination of the PO4 tetrahedra in the two phases: in the olivine phase, the P-O stretching vibration alsoinvolves, to a high degree, the stretching of Ni-O bonds, whereas in the β′ phase, the ν1 mode involves, together with the P-O stretching, the bending of the O-Ni-O angles. Electrochemical Behavior. The effect of the olivine S β′-LiMPO4 phase transition on the electrochemical properties of these materials is clearly observed in Figure 10. As previously reported,7 among the LiMPO4 (M ) Mn, Fe, Ni) olivines, only the iron-containing compound is electrochemically active within the stability window of the electrolyte (from 0.0 to 5.1 V). From the olivine LiFePO4, up to 0.8 lithium per unit formula can be reversibly extracted at about 3.7 V, whereas no oxidation results for the β′-LiMPO4 materials. In this connection, the LiCoPO4 olivine has very recently been reported to be a very promising high-voltage cathodic material.8 It is well-known that an electrochemical cell transforms chemical energy into electrical energy. It consists of a material at the anode that is the reductant of the chemical reaction and an oxidant at the cathode. Both the reductant and the oxidant are consumed on discharge of the battery and produced on charge. The energy of their reaction divided by the electronic charge produced in the reaction gives the maximum discharge voltage of the cell. It must be recalled that raising the energy of a given redox couple in a cathode lowers the voltage of the cells using a common anode, whereas raising the redox energy of an anode raises the cell voltage for a given cathode. Because the energy of a given redox couple depends on the electrostatic field at the cation position and the covalent contribution to the cation-anion bonding,7 the different behavior observed in the iron-containing compounds (olivine and β′ phase) can be explained by the change in the value of the electrostatic field at the iron position produced by the phase transition. A Madelung calculation yields the values shown in Table 8. Thus, as a consequence of the

β′-LiFePO4

atom

electrostatic field (V)

Fe Li P O(1) O(2) O(3)

-27.3 -14.9 -53.1 16.9 27.8 27.4

atom

electrostatic field (V)

Fe Li P O(1) O(2)

-26.7 -15.3 -59.6 28.7 27.4

phase transition, a significant stabilization is produced in the redox energy of the couple Fe3+/Fe2+, from -27.5 eV in the olivine phase to -26.7 eV in β′-LiFePO4. Therefore, a significant increase of the cell voltage is expected.7 Indeed, no electrochemical processes appear on charging β′-LiFePO4 up to the upper limit of the experimental setup. In addition to these considerations, lithium mobility in the two structures must be considered, as high lithium mobility might favor good electrochemical performance. Because the lithium ions in the olivine structure are located in octahedral sites sharing edges, whereas in β′-LiFePO4 they are in isolated tetrahedral positions, the lithium mobility in the former structure is expected to be much higher than that in the latter. However, the measured conductivity of LiNiPO4 olivine is very low, above 722 K, the resistance of the pellet is too high to obtain accurate data. Thus, the ionic conductivity of LiNiPO4 olivine at 772 K is as low as 10-8 Ω-1 cm-1. On the other hand, the resistance of the pellet of β′-LiNiPO4 is too high to be measured with the experimental setup; also, because the β′-phase transforms at room pressure to the olivine phase at about 697 K, it was not possible to determine the mobility of lithium in this material. Conclusions Olivine-like phosphates LiMPO4 (M ) Fe and Ni) undergo a phase transition under high pressure, whereas the manganese-containing compound seems to be stable, at least up to 65 kbar. Instead of the well-known olivine S spinel transition, a transformation into a new phase is produced (β′ phase). The main difference between the two structures is the oxygen arrangement, as the metallic frameworks are similar. Surprisingly, the coordination of lithium is modified from 6- (in olivine) to 4-fold (in β′) under pressure. The structural model proposed for the β′ phase has been confirmed by Raman spectroscopy. Among the olivines LiMPO4 (M ) Mn, Ni, and Fe), only the iron-containing one is electrochemically active within the electrolyte stability window (up to 5.1 V), whereas none of the high-pressure materials is active. This can be explained by the change in the electrostatic field at the transition metal position induced by the phase transition. Acknowledgment. This study was financially supported by CICYT (Project MAT 98-1053-C04-04), CAM (Project CAM-07N/0012/1998), and USP (Project USP USP-7/99). CM000596P